On Tuesday, the Food and Drug Administration (FDA) approved Qsymia, the second new diet drug in a month, and the most effective of the weight-loss pills that the agency has considered in recent years.

Qsymia, made by Vivus Inc., receives market approval on the heels of Arena’s Belviq (lorcaserin), the diet pill cleared by the FDA in late June. Before these two new drugs, the last prescription weight-loss pill to be green-lit by the government was Roche’s Xenical (orlistat) 13 years ago, in 1999.

Diet pills have had a spotty history with the FDA, largely because of safety problems involving the heart. Other drugs — most notably fenfluramine, used in the popular weight-loss combination fen-phen — have been withdrawn from the market for such hazards, and new drugs have had to clear a high bar for consideration.

The new pill, formerly known as Qnexa (the FDA asked Vivus to change the name to avoid confusion with another drug on the market), will be available by the fourth quarter of this year, according to the company’s president, Peter Tam. Here’s what you need to know.

Who can use Qsymia?
The drug is approved for obese adults with a body mass index, or BMI, of 30 or higher. It can also be used by overweight adults, with a BMI of 27 or higher, if they have at least one weight-related condition such as high blood pressure, Type 2 diabetes or high cholesterol.

How does it work?
Qsymia is a combination of two existing drugs: phentermine, an appetite-suppressing stimulant that has long been used for short-term weight loss, and topiramate, an anti-seizure medication used to treat epilepsy that makes people feel fuller after eating. Some doctors have already been prescribing the two drugs together for weight loss. Researchers say the key to Qsymia’s success is that it targets multiple brain pathways that trigger overeating.

How much weight will I lose?
It varies. In clinical trials, overweight and obese patients taking Qsymia for a year lost differing amounts of weight: on average, patients taking a middle dose of the drug lost 8.4% of their body weight; on a higher dose, patients lost 10.6%.

Most of the weight came off in the first three months, so doctors should monitor patients at that point to see if the drug is working. The FDA notes that people who don’t lose at least 3% of their body weight by three months are unlikely to go on to lose any significant weight, so they should either be counseled to discontinue the drug or to try a higher dose (Qsymia will be available in two doses). If patients still don’t lose at least 5% of their weight after three additional months on the higher dose, they should quit taking Qsymia.

The drug is designed to be used in conjunction with traditional weight-management strategies like diet and exercise. “This is not a magic pill. Patients can’t take it and think that’s all they have to do. It needs to be used with a lifestyle modification program,” W. Timothy Garvey, chair of the department of nutrition sciences at the University of Alabama-Birmingham, who studied Qsymia, toldUSA Today.

What if I just want to drop a few pounds?
Doctors stress that because of the potential risks of the drug, dieters should not use the drug for cosmetic weight loss. “It’s not for patients who want to lose a few pounds,” Tam toldUSA Today. Of course, concerns remain that once the drug is on the market, doctors can prescribe it to all comers. To prevent that, Qsymia will be available only by mail order at first, so that physicians can’t dispense it from their offices. It will also be available in specialized pharmacies that register for the right to sell the drug, where pharmacists have been educated about Qsymia’s risks and can pass that information along to patients and doctors.

What are the risks?
Potential heart problems and birth defects. In trials, some people taking Qsymia had an increased heart rate as well as a condition called metabolic acidosis, which can lead to hyperventilation, fatigue and anorexia. One of the ingredients in Qsymia, topiramate, has also been linked in previous studies to birth defects including cleft lip and cleft palate in women who take it during pregnancy.

Safety concerns led the FDA to reject Qsymia’s first bid for approval in 2010, but the agency and Vivus have now put in place strategies to reduce risk: for example, women of child-bearing age who want to take Qsymia must test negative for pregnancy before starting the drug and are expected to use contraception and take a pregnancy test once a month while on the drug. Worries remain, though: in a Vivus trial, 34 women on Qsymia became pregnant even though they were instructed to use contraception. (The company says no birth defects occurred.)

Patients’ heart rate should also be monitored regularly while taking Qsymia. People with recent or unstable heart disease or stroke are not recommended to take the drug because of the potential heart risks. Vivus has also agreed to continue monitoring Qsymia users for side effects after the drug reaches market; in particular, it will conduct a long-term cardiovascular outcomes trial to assess the effect of Qsymia on major events like heart attack and stroke.

Which is better, Qsymia or Belviq?
In clinical trials, people taking Qsymia lost more weight; those taking Belviq lost 5.8% of their body weight on average after a year. Based on the side effects profiles, however, some might consider Belviq to be less risky. “The majority of obese patients wanting to try drug therapy will end up seeing both drugs, regardless of which one they take first,” Simos Simeonidis, an analyst at Cowen & Company, told the New York Times, noting that he thought Qsymia would sell better.

Why is this approval important?
Because more than a third of American adults are obese — a proportion that’s growing — and there aren’t many effective weight-loss treatments out there to reverse the trend. Aside from diet and exercise or weight-loss surgery, patients have few options. The FDA’s approval of Qsymia, after such a long diet-drug drought and despite the potential safety problems that plague weight-loss pills, marks a willingness to make new solutions available.

How do you pronounce the name of this pill?
Qsymia is pronounced cue-SIM-ee-uh.